Electric discharge lamps such as metal halide lamps which include mercury, the metals of various halides and particularly sodium iodide undesirably are subject to electrolysis between the usual starter electrodes and the electrode immediately adjacent thereto. Moreover, any DC potential existing between the above-mentioned electrodes undesirably increases electrolytic activity and especially so as operational temperatures increase.
Generally, the metal halide lamps include a sealed glass envelope with a fused silica arc tube disposed within the sealed glass envelope. For some time, the outer glass envelope was evacuated whereupon a thermal switch was subjected to a temperature which was determined by lamp wattage and substantially independent of lamp orientation or position of operation. Thus, a simple thermal switch could be incorporated into the outer glass envelope and accommodate temperature variations encountered while employing commercially available bimetal materials.
As an example of the employment of a relatively simple thermal switch in a metal halide discharge lamp, reference is made to U.S. Pat. No. 3,226,597, issued Dec. 28, 1965, in the name of A. C. Green. Therein, a substantially U-shaped bimetal strip expands, in response to heat, to contact an inlead (16) and short-circuit the inleads 16 and 18 respectively.
However, better performance and other desirable features and considerations led to the use of a gas fill within the outer glass envelope. Accordingly, the variable of convective heating was added to lamp wattage in determining the operating temperatures of a thermal or bimetal type switch. Moreover, lighting fixture manufacturers, in recent years, are designing systems to widen the beam spread of available lamps and have utilized the concept of moving the lamp further into the reflector cavity of the fixture. As a result, the temperature of the lamp components and of the thermal switch associated with the lamp has been increased. Unfortunately, sucn increased temperatures tend to stress the bimetallic material of available thermal switches beyond the elastic limit whereupon permanent deformation of the thermal switch undesirably results. Thus, thermal switch failure permits the previously-mentioned electrolysis to take place and results in failure of the discharge lamp.
Additionally, the above-mentioned overheating of the thermal switch presents no problem with regard to deformation when a switch is used which is normally closed and opens as the temperature is increased. However, for normally open switches which close as the temperature increases such permanent discharge of the switch is a problem. Therein, contact closure imposes a restraint in movement of the bimetal material, and this restraint may cause a permanent deformation which prevents return of the switch to a normally open condition and eventually results in lamp failure.
One known attempt to overcome the above-mentioned permanent deformation of the bimetal material in a discharge lamp is set forth in U.S. Pat. No. 3,965,387, issued to Stuart et al on June 22, 1976. Therein, a spring-like member is affixed to one end of a bimetal strip. The bimetal strip is fastened at the opposite end to an electrical conductor of the discharge lamp. Upon activation due to heat, the bimetal flexes which, in turn, causes the spring-like member to contact a second electrical conductor. Thus, short-circuiting of a pair of electrical conductors is effected. Moreover, the spring-like member deflects which prevents distortion of the bimetal strip.
Although the above-described structure has been and still is employed in various forms of discharge lamps, it has been found that this particular structure does leave something to be desired. More specifically, it has been found that individual attachment of a bimetal strip and of a spring-like member permits a precise setting of the gap therebetween which, in turn, enhances control over the operation of the structure. Moreover, this enhanced operational control is achieved without increased cost and with a reduction in complexity of the structure.